1. Field of the Invention
The present invention relates to a pulse operating method for a sensor, and more particularly, to a sensor operating method of applying a pre-bias voltage pulse to an FET (Field Effect Transistor)-type sensor having a floating gate formed in a horizontal direction to be capable of improving sensitivity and recovery characteristics of the sensor and allowing the sensor to operate with low power.
2. Description of the Related Art
In recent years, as the demand for sensors is increased, various types of sensors have been developed. Among these sensors, FET (Field Effect Transistor) sensors having a floating gate with high input impedance and high amplification rate have been researched. In addition, as low power and high sensitivity due to nigh transconductance and incorporation with existing CMOS circuits are required, FET-type sensors have been increasingly researched.
Korean Patent Laid-open Publication Ho. 2013-52528 (Patent Document 1) relates to an “FET-type gas sensor having a horizontal floating gate”. In Patent Document 1, disclosed is an FET-type gas sensor where a control electrode and a floating electrode are formed in a horizontal direction with a sensing material layer interposed therebetween in order to solve problems occurring in the existing FET-type sensors where a control electrode, a floating electrode, and a sensing layer are formed in a vertical direction. The problems occurring in the existing FET-type sensors are a low coupling ratio between the control electrode and the floating electrode, low sensitivity, and high power consumption caused from limitation in selecting a sensing material and a parasitic capacitance component and high manufacturing cost caused from process complexity. In addition, several sensing mechanisms depending on a structure of the sensor are disclosed, and arrays including a plurality of gas sensors operating with the sensing mechanisms are also disclosed.
FIGS. 1A to 1D are cross-sectional views of an FET-type gas sensor having a horizontal floating electrode capable of sensing a change in work function as disclosed in Patent Document 1. Since a control electrode is directly electrically connected to a sensing material layer, a voltage of the sensing material layer is changed together with a voltage of the control electrode according to a change in voltage of the control electrode. When the work function of the sensing material layer reacting with a specific gas is changed, the work function of the control electrode is changed. Therefore, the voltage to be transferred to the floating electrode is changed, and channel formation and channel resistance of the semiconductor body are affected. By using this point, the specific gas is sensed with a current flowing through source and drain electrodes.
FIGS. 2A and 2B are a plan view and a cross-sectional view of an FET-type gas sensor having a horizontal floating electrode capable of sensing a change in work function and a change in capacitance disclosed in Patent Document 1. Unlike the gas sensor disclosed in FIGS. 1A to 1D, the control electrode and the sensing material layer are formed to be separated from each other with an insulating material interposed therebetween. Accordingly, the voltage of the control electrode is transferred to the sensing material, layer by the capacitance of the control electrode and the sensing material layer, and the voltage of the floating electrode is determined by the capacitance of the sensing material layer and the floating electrode.
The sensing material layer reacts with a specific gas to change a dielectric constant or a work function, so that a voltage to be transferred to the floating electrode is changed. Therefore, by using the characteristic that the channel formation and the channel resistance of the semiconductor body are affected, it is sensed by a change in current flowing through the source and drain electrodes.
FIGS. 5A to 5C illustrate cross-sectional views of an FET-type gas sensor that can be used as a heater by applying a voltage across a first electrode. An air layer is formed with a certain depth in the semiconductor substrate below the first electrode and the sensing material layer, and thus, heat is prevented from being released through the semiconductor substrate having high thermal conductivity, so that it is possible to effectively transfer heat to the sensing material layer. By applying a predetermined voltage to both ends of the first electrode, a current flows to generate heat, so that it is possible to improve reactivity between the sensing material layer and a gas.
Korean Patent Laid-Open Publication No. 2014-106335 (Patent Document 2) relates to a “three-dimensional Fin-FET-type gas sensor having a horizontal floating gate”. In Patent Document 2, similarly to Patent Document 1, disclosed is a three-dimensional Fin-FET-type gas sensor where s floating electrode is formed to surround a semiconductor body protruding in a FIN shape to enlarge a width of a channel to increase a drain current, so that, the Fin-FET-type gas sensor has an advantage of increasing the sensitivity of the sensor.
Non-Patent Document 1 is “Highly selective ZnO gas sensor based on MOSFET having a horizontal floating-gate, Sensors and Actuator B”, Y. Hong et al., Chemical, 232, p 653, 2016. In Non-Patent Document 1 gas reaction is sensed by using an FET-type gas sensor where a control electrode and a floating electrode are formed in a horizontal direction with a sensing material layer interposed therebetween to sense a change in work function disclosed in Patent Document 1. When an oxidizing and a reducing gas react with the sensing material layer, the work function of the sensing material layer is changed. As a result, a threshold voltage of the FET-type gas sensor is changed, and thus, a change in drain current is sensed. A certain read voltage (read-bias voltage pulse is applied to the gate, source, and drain to check gas reactivity caused from a change in source/drain current according to presence or absence of gas adsorption. However, when a constant voltage (DC voltage) is applied in the measurement of the gas reaction in the FET-type gas sensor, the drain current continues to flow, and thus, stress and power consumption of the gas sensor is increased.
Non-Patent Document 2 is “Highly improved response and recovery characteristics of Si FET-type gas sensor using pre-bias”, J. Shin et al., International Electron Device Meeting. In Non-Patent Document 2, as an example of a pulse operating method for an FET-type gas sensor having a horizontal floating gate according to the invention, the reactivity and the recovery time of the sensor is greatly improved in comparison with the result of the measurement through the DC voltage application in Non-Patent Document 1. By applying a negative pre-bias voltage pulse to the control electrode, the reaction of NO2 as an oxidative gas is improved, and by applying a positive pre-bias voltage pulse, the recovery time is greatly reduced due to facilitation of desorption of the oxidizing gas. The source voltage is always maintained at 0 V, and the pulse applied to the drain electrode is synchronized with the pulse applied to the control electrode. The source voltage is always maintained at 0 V, and while the pre-bias voltage pulse is applied to the control electrode, a voltage of 0 V is applied to the drain electrode to prevent a current from flowing in the gas sensor, so that power consumption is reduced.
Accordingly, the invention proposes a pulse operating method for an FET-type gas sensor having a horizontal floating electrode and a sensor array manufactured by using the FET-type gas sensor capable of improving gas reaction and recovery characteristics and reducing power consumption. In addition, the invention proposes a biasing method capable of effectively operating locally-implemented heaters by using characteristics of voltage pulse application.